The present invention relates to the control of the air/fuel ratio, AFR, in fossil fired furnaces such as those normally used in steam boilers. More particularly this invention relates to the control of the combustion air flow in the firing of such a furnace so as to maintain the heat losses to the stack at a minimum, thus optimizing the combustion air flow.
It is desirable to carry out this control under varying operating conditions, such as:
1. Variations in fuel quality PA1 2. Variations in the fuel-air mixing with load PA1 3. The presence of infiltrated air PA1 4. Burner fouling PA1 5. The use of multiple fuels
In theoretically perfect or stoichiometric combustion there is a complete reaction of all of the fuel and oxygen without unburned fuel or unreacted oxygen remaining. The last step in such a perfect combustion process is the disappearance of CO, which is consumed in combustion. As a practical matter perfect combustion is not possible and there is always a remaining quantity of CO and other combustibles such as hydrogen and particulate in the exit gases along with an excess of oxygen in the form of excess air. The presence of this excess air and the presence of the combustibles causes an increase in the stack heat losses since the heat content of the combustibles is not realized and the air as well as the combustibles must be brought up to the exit gas temperature. Thus, efficiency can be greatly affected by the quantity of excess air.
In the past, when cheap fuels were available, combustion control systems operated with a bias toward the region of excess air, preferring the small cost penalty associated with excess air as contrasted to the steep penalty associated with high CO operation. Consequently, control of the combustion air from excess oxygen has become the standard for combustion control systems. Among the limitations of the oxygen measurement are the fact that it is not a direct indicator of complete combustion. Under certain conditions excess oxygen and CO can coexist in the combustion products. This can occur, for example, due to stratification and air infiltration. Stratification arises due to incomplete mixing of combustibles. Air infiltration is also bad for oxygen measurements because the oxygen in infiltrated air causes a large error in the combustion products analysis so that the control system can be seriously misled. Thus, it will be evident that under certain conditions the oxygen measurement is not an indicator of complete combustion and the presence of unburned fuel cannot be judged on the basis of the amount of oxygen in the flue gases.
The CO measurement, unlike the oxygen measurement, is a direct indicator of complete combustion, however, as with oxygen the CO measurement is affected by infiltrated air, but not as much. The use of CO to control air/fuel ratio will control the level of unburned products, but may cause the use of uneconomical amounts of excess air. For example, if the burner gets dirty or there is poor mixing, control from CO will increase the excess air and may actually decrease fuel burning efficiency. Control from oxygen may allow, under the same conditions, an increase in combustible content of the flue gas. Thus, neither approach solves the problem of obtaining efficient combustion.
Some recent attempts have been made to use a combination of the oxygen and the CO measurements to obtain control of the air/fuel ratio so as to provide efficient operation. These have included the system mentioned by Alfred Watson in an article entitled, THE CO--O.sub.2 --CO.sub.2 RELATIONSHIP IN COMBUSTION CONTROL. In that article it is proposed to use oxygen under dynamic conditions to maintain the fuel/air ratio at a state where the carbon monoxide value does not exceed 1000 ppm. High and low set points are provided for the oxygen controller which are approximately equivalent to the upper and lower CO values. Under steady state conditions CO controls the air flow. The set point is approximately 150 ppm. The system is so arranged that the high and low oxygen limits apply, even under steady state control. This method, however, would not manage to keep operation at maximum efficiency.
It is an object of this invention to overcome the problems inherent in these prior art systems and provide a control of the air/fuel ratio such that there is a minimum heat loss.